Page 229 - Mechanical Engineer's Data Handbook

MANUFACTURING TECHNOLOGY 217

Types of .fit 5.13.2 Selected Fits
The fit describes the manner in which two parts go BS 4500 ‘Selected Fits’ Gives a much smaller range of
together. A ‘clearance fit’ means that the shaft will fits, the hole tolerance is denoted by the letter H and
always be smaller than the hole. An ‘interference fit’ the shaft by a lower-case letter (see table). For
means that the shaft will always be larger than the hole conventionally manufactured parts, the five fits given
and a fitting force will be necessary. A ‘transition fit’ are usually sufficient (see table).
means that there may be either clearance or interfer-
ence. selected fits (Bs 4500)
Tolerance Hole H7 H8 H9 H11

BS 4500 gives 18 ‘tolerance grades’ numbered ITO1, Shaft cll d10 e9 fl g6 h6 k6 n6 p6 s6
ITO, IT1, IT2, up to IT16. The actual tolerance
depends on the size of the component (see table
below).

Reduced range of fits for conventionally manufactured prts
_______~ ~~
Type of fit Shaft tolerance Hole tolerance Description of fit

Clearance fl H8 Running
Clearance g6 H7 Sliding
Transition k6 H7 Keying
Interference P6 H7 Press
Interference s6 H7 Push or shrink

5.13.3 Example of symbols and sizes on On the drawing these parameters would be given as
drawing (rounding off to nearest 0.01 mm):
Hole: 30.01 Shaft: 30.02
Preliminary design drawing
30.00 30.00
It is convenient to use symbols, e.g. 45 mm shaft and
‘transition’ fit. Tolerance is given as: 4 45H7/k6.

Production drawing

For a 30mm diameter shaft, fit H9/d10:
Hole maximum limit of size= 30.012 mm
Hole minimum limit of size = 30.00 mm
Therefore tolerance = 0.012 mm.
Shaft maximum limit of size = 30.015 mm
Shaft minimum limit of size = 30.002 mm

Therefore, tolerance = 0.01 3 mm